Tunable negative bitline write assist and boost attenuation circuit

ABSTRACT

An apparatus and method are provided for implementing write assist with boost attenuation for static random access memory (SRAM) arrays. The apparatus includes a memory array comprising a plurality of SRAM cells. The apparatus further includes a write driver connected to each of a differential pair of bit lines in each of the plurality of SRAM cells of the memory array. The apparatus further includes a write assist attenuation circuit connected to the write driver, the write assist attenuation circuit comprising a clamping device configured to modify a control signal as a function of supply voltage and process to attenuate an amount of boost applied to pull one of the bit lines below ground in an active phase of a write cycle.

FIELD OF THE INVENTION

The invention relates to memory devices and, more particularly, to anapparatus and method for implementing write assist with boostattenuation for static random access memory (SRAM) arrays.

BACKGROUND

Memory devices are commonly employed as internal storage areas in acomputer or other type of electronic equipment. One specific type ofmemory used to store data in a computer is random access memory (RAM),such as static RAM (SRAM) or dynamic RAM (DRAM), for example. RAM istypically used as main memory in a computer environment. RAM isgenerally volatile, in that once power is turned off, all data stored inthe RAM is lost.

A typical SRAM device includes an array of individual SRAM cells. EachSRAM cell is capable of storing a binary voltage value therein, whichvoltage value represents a logical data bit (e.g., “0” or “1”). Oneexisting configuration for an SRAM cell includes a pair of cross-coupleddevices such as inverters. With complementary metal oxide semiconductor(CMOS) technology, the inverters further include a pull-up PFET(p-channel) transistor connected to a complementary pull-down NFET(n-channel) transistor. The inverters, connected in a cross-coupledconfiguration, act as a latch that stores the data bit therein so longas power is supplied to the memory array. In a conventionalsix-transistor (6T) cell, a pair of access transistors or pass gates(when activated by a word line) selectively couples the inverters to apair of complementary bit lines. Other SRAM cell designs may include adifferent number of transistors, e.g., 4T, 8T, etc.

The design of SRAM cells has traditionally involved a compromise betweenthe read and write functions of the memory cell to maintain cellstability, read performance, and write performance. The transistorswhich make up the cross-coupled latch must be weak enough to beoverdriven during a write operation, while also strong enough tomaintain their data value when driving a bit line during a readoperation. The access transistors that connect the cross-coupled cellnodes to the true and complement bit lines affect both the stability andperformance of the cell. In one-port SRAM cells, a single pair of accesstransistors is conventionally used for both read and write access to thecell. The gates are driven to a digital value in order to switch theaccess transistors between an on and off state. The optimization of anaccess for a write operation would drive the reduction of theon-resistance (R_(on)) for the device. On the other hand, theoptimization of an access transistor for a read operation drives anincrease in R_(on) in order to isolate the cell from the bit linecapacitance and prevent a cell disturb.

One approach to improving write performance of SRAM devices is to useso-called “negative boosting” to discharge a bit line to a voltage levelbelow the nominal low supply rail value (e.g., ground). In so doing, theaccess transistors or pass gates of the SRAM cell coupled to thedischarged bit line see a resultant increase in both the gate-to-sourceand drain-to-source voltages. This negative boosting facilitatesinternal node discharge and flipping of the cross-coupled latch ascompared to more conventional write techniques in which the bit line issimply discharged to the value of the nominal low voltage rail (e.g.,ground).

SUMMARY

In a first aspect of the invention, a device is provided for thatincludes a memory array comprising a plurality of SRAM cells. The devicefurther includes a plurality of true bit lines each connected to acolumn of the memory array. The device further includes a plurality ofcomplement bit lines each forming a differential pair with, and in asame column as, one of the plurality of true bit lines. The devicefurther includes a write driver connected to each of the differentialpair of bit lines in each of the plurality of SRAM cells of the memoryarray. The write driver includes a negative boost node and a dischargedevice coupled to ground and the negative boost node. The dischargedevice is configured to receive a control signal and pull one of theplurality of true bit lines or one of the plurality of complement bitlines to ground in an active phase of a write cycle. The write driverfurther includes a boost capacitor coupled to the negative boost node.The boost capacitor configured to boost the one of the plurality of truebit lines or the one of the plurality of complement bit lines belowground. The device further includes a write assist attenuation circuitconnected to the discharge device, the write assist attenuation circuitcomprising a clamping device configured to modify the control signal asa function of supply voltage and process to attenuate an amount of theboost.

In another aspect of the invention, a SRAM write assist attenuationcircuit is provided for that includes a clamping device comprising afirst NFET, a second NFET, and a third NFET connected to a controlsignal. The SRAM write assist attenuation circuit further includes alogic structure comprising one or more OR gates configured to generate afirst attenuation signal, a second attenuation signal, and a thirdattenuation signal. The first attenuation signal, the second attenuationsignal, and the third attenuation signal are configured to individuallyactivate either the first NFET, the second NFET, or the third NFET suchthat only one of the first NFET, the second NFET, and the third NFETmodify the control signal as a function of a supply voltage and aprocess to attenuate an amount of boost applied to pull one of aplurality of true bit lines or one of a plurality of complement bitlines below ground in an active phase of a write cycle.

In a further aspect of the invention, a SRAM device is provided for thatincludes a memory array comprising a plurality of SRAM cells. The SRAMdevice further includes a plurality of true bit lines each connected toa column of the memory array. The SRAM device further includes aplurality of complement bit lines each forming a differential pair with,and in a same column as, one of the plurality of true bit lines. TheSRAM device further includes a write driver connected to each of thedifferential pair of bit lines in each of the plurality of SRAM cells ofthe memory array. The SRAM device further includes a write assistattenuation circuit connected to the write driver. The write assistattenuation circuit includes a clamping device comprising a first NFET,a second NFET, and a third NFET connected to a control signal, The writeassist attenuation circuit further includes a logic structure configuredto generate a first attenuation signal, a second attenuation signal, anda third attenuation signal. The first attenuation signal, the secondattenuation signal, and the third attenuation signal are configured toindividually activate either the first NFET, the second NFET, or thethird NFET such that only one of the first NFET, the second NFET, andthe third NFET modify the control signal as a function of a supplyvoltage and a process to attenuate an amount of boost applied to pullone of the plurality of true bit lines or one of the plurality ofcomplement bit lines below ground in an active phase of a write cycle.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows a simplified block diagram of a memory array in accordancewith aspects of the invention;

FIG. 2 shows a SRAM write driver in accordance with aspects of theinvention;

FIG. 3 shows a SRAM write assist circuit implemented with a SRAM writedriver and memory cell in accordance with aspects of the invention;

FIG. 4 shows a circuit timing diagram that describes the operation ofthe SRAM write assist circuit depicted in FIG. 3 in accordance withaspects of the invention;

FIG. 5 shows a SRAM write driver along with a write assist attenuationcircuit in accordance with aspects of the invention;

FIG. 6 shows a SRAM write assist attenuation circuit in accordance withaspects of the invention;

FIG. 7 shows a logic table that describes the operation of the SRAMwrite assist attenuation circuit depicted in FIG. 6 in accordance withaspects of the invention;

FIG. 8 shows a SRAM write driver in accordance with aspects of theinvention; and

FIGS. 9, 10A, 10B, and 11 show circuit timing diagrams that describe theoperation of the SRAM write assist attenuation circuit depicted in FIG.6 in accordance with aspects of the invention.

DETAILED DESCRIPTION

The invention relates to memory devices and, more particularly, to anapparatus and method for implementing write assist with boostattenuation for SRAM arrays. More specifically, implementations of theinvention provide for a negative boost write assist attenuation circuitwith a clamping device that is configured to provide boost attenuationat higher supply voltages and faster process (e.g., in accordance withaspects of the present invention a higher supply voltage is understoodto mean any voltages that are greater than the technology nominalvoltage, and a faster process means devices operating faster than thetechnology nominal process) with less attenuation at lower supplyvoltages based on supply voltage and processes. In this manner,attenuation with clamping proportional to supply voltage and processesgives exponentially increased attenuation because of larger leakage,which advantageously provides a tunable negative boost write assistattenuation circuit that can either reduce attenuation or increaseattenuation at particular voltages.

FIG. 1 shows a simplified block diagram of a memory 100 that includes amemory array 105. Memory array 105 comprises memory cells, such asmemory cell 110, in which bits are stored. For ease of illustration,FIG. 1 shows only memory cell 110, however, memory array 105 would havememory cells arranged in each of its rows and columns. In embodiments,memory 100 is a SRAM. Although the description that follows for FIG. 1and other figures is directed to an SRAM, those skilled in the art willrecognize that embodiments described below are also suited for use withother memory devices such as DRAM. A typical SRAM cell includes abalanced pair of cross-coupled inverters storing a single data bit. Apair of pass gates (a balanced pair of field-effect transistors (FETs))selectively connects the complementary outputs of the cross-coupledinverters to a corresponding complementary or differential pair of bitlines (e.g., bit line true and bit line complement). A word lineconnected to the gates of the pass gate FETs selects the memory cell 110to the corresponding complementary pair of bit lines for performing anoperation that may include a read or a write operation. The SRAM cellmay be formed from cell designs that may include any number oftransistors (e.g., 4T, 6T, 8T, etc.).

FIG. 1 does not illustrate a particular SRAM cell design; however, FIG.1 shows a bit line 115 (representing bit line true and bit linecomplement) connecting to memory cell 110 through a column of memoryarray 105 and a word line 120 connecting to memory cell 110 through arow of memory array 105. As an example, an N row by M column SRAM arraymay be organized as N word lines by M column bit lines. In operation,the bit line pairs (e.g., bit line true and bit line complement)represented by bit line 115, are in standby and are clamped together toa supply or reference voltage. Accessing bit line 115 for a read or awrite operation from memory array 105 entails driving one of the N wordlines, e.g., turning on the pass gates for all memory cells 110 on thatword line. With the pass gates on for that selected word line, thecross-coupled cell inverters are coupled to the corresponding bit linepairs, partially selecting the cells on that word line. Selection of oneof the M columns selects the cells on that word line, with the bit linesof a particular cell actually being accessed. The remaining bit lines ofthe other cells in the column remain half selected during the access.

Those skilled in the art will recognize that memory cell 110 may includeother elements than what is illustrated in FIG. 1. For example, memorycell 110 may include a sense amplifier for facilitating read operationsand a write driver for facilitating write operations.

FIG. 2 shows a conventional SRAM write driver 200 that may beimplemented in memory cell 110 of FIG. 1. The SRAM write driver 200 iscoupled to the bit line pairs (e.g., bit line true and bit linecomplement) represented by bit line 115 via bit switches (not shown).The bit line true may be formed from one or more true bit lines and thebit line complement may be formed from one or more complement bit lines.The bit switches, which are used to connect one of the bit lines (e.g.,bit line true or bit line complement) to SRAM write driver 200, areformed from N-type field effect transistors (NFETs) with gates connectedto the one or more true bit lines for bit line true and similar NFETswith gates connected to the one or more complement bit lines for bitline complement. Note that cross-coupled inverters described above foran SRAM cell would be coupled to each of the bit lines.

In a typical write operation, after a particular word line has beenactivated, one of the bit lines (e.g., bit line true) is brought toground (GND), while the other bit line (e.g., bit line complement) ismaintained at a supply voltage (VDD). This discharges the cell andfacilitates a write to the cell. In particular, this enables the “1”side (e.g., bit line true) to change to a “0”, allowing the cell to flipstates. As shown in FIG. 2, a discharge device TD (an NFET) and a boostcapacitor C_BOOST are used to discharge one of the bit lines (e.g., bitline true or bit line complement) below the nominal low supply railvalue (e.g., ground). Capacitor C_BOOST is coupled to discharge deviceTD at a node WBOOST. The gate of discharge device TD, which iscontrolled by signal WSELP<0>, is coupled to capacitor C_BOOST via NFET205 and PFET 210. Discharge device TD and capacitor C_BOOST areconnected to bit switches, bit line true, and bit line complement viaNFETs 215 and 220, which are controlled by data signals WGDLT and WGDLC,respectively.

However, notwithstanding the benefits of negative boosting, it has beendetermined that in instances where the supply voltage (VDD) is high(e.g., any voltages that are greater than the technology nominalvoltage), the negative boosting may create stress within the SRAM writedriver. For example, the gates of the FETs in the SRAM write driver areat VDD, while their sources go negative due to their coupling to thenegative voltage node. If VDD is at about 1.1V (e.g., a higher VDD)while the source of one of these FETs goes to −300 millivolts (mV)during a boost below GND, then a 1.4V differential may arise at thegate-source terminal of the transistors. Over time it has beendetermined that such voltage levels create stress on the write driver(e.g., affect the gate oxide of the transistors) causing write driverreliability concerns.

FIG. 3 shows a conventional write assist circuit 300 that may beimplemented in SRAM write driver 200 of FIG. 2 to overcome stressimparted on the writer driver by the negative boosting. The write assistcircuit 300 enables SRAM write driver 200 to provide a full boost belowGND at lower VDD levels; however, as the VDD levels increase, the writeassist circuit 300 attenuates or limits the amount of boost. The writeassist circuit 300 performs these functions by controlling a timingrelationship between the discharge device TD and the capacitor C_BOOST.In particular, the write assist circuit 300 enables discharge device TDto shut off prior to initiating the boost provided by capacitor C_BOOSTand slow the shutting off of the discharge device TD as the supplyvoltage VDD increases.

As shown in FIG. 3, the write assist circuit 300 comprises a boostcontrol device 305 configured to control the amount of boost provided bycapacitor C_BOOST as a function of VDD. A discharge device controller310 is the part of the write assist circuit 300 that controls operationof discharge device TD, such that the operational relationship betweendischarge device TD and capacitor C_BOOST is timed to provide the fullboost provided by capacitor C_BOOST to the bit line at lower VDDvoltages, while at higher VDD voltages, attenuate the amount of boost.The discharge device controller 310 comprises a bias signal generator315 (e.g., a resistor based BIAS scheme) that generates a WBIAS signalthat is provided as function of VDD. The bias signal generator 315comprises a stack of NFETs 320 coupled in series.

The discharge device controller 310 further comprises a switching stage325 that is configured to control the timing of the shutting off of thedischarge device TD in accordance with the BIAS signal generated frombias signal generator 315. In particular, switching stage 325 slows downthe timing of shutting off discharge device TD (i.e., slows thetransition of signal WSELP<0>) in the presence of a high VDD voltage.The switching stage 325 comprises a PFET 330 and two NFETs 335 and 340coupled in series. In particular, the source of PFET 330 is coupled tosupply voltage VDD, its drain is coupled to the drain of NFET 335 andits gate is controlled by signal WBLN. NFET 335 is controlled by theBIAS signal and its source forms a node NS with the drain of NFET 340and the source of NFET 345. The gate of NFET 345 is controlled by theWBLN signal and the drain of NFET 345 is coupled to VDD. A node 350 isformed between the drain of PFET 330 and an input of SRAM write driver200.

In operation, when the WBLN signal goes high, then NFET 335 is impactedbecause the WBIAS voltage is lowered and the voltage at the node 350 isincreased. Essentially, because the node 350 is at VDD and the node NSis connected through NFET 345, more time is needed to discharge the node350 to GND. The end result is that the switching provided by switchingstage 310 slows down as VDD increases. This creates the timingrelationship necessary to generate a response from additional logic (notshown) that keeps the WSELP<0> signal high during the boost. Asmentioned above, this enables the boost at the higher VDD voltages to beattenuated through the discharge device TD.

FIG. 4 shows a simulation 400 of the discharge of a bit line (BLT) withthe write assist circuit 300 depicted in FIG. 3. In particular,simulation 400 shows the transition of a bit line (BLT) from high tolow, and then to the point where it receives a boost (WBOOST) below GND(e.g., 0 volts). As shown in FIG. 4, at lower VDD voltages (e.g., 0.7volts and 0.8 volts), signals WSELP<0:1> transition at different times.This happens because the SRAM write assist circuit 300 is controllingthe timing between discharge device TD and capacitor C_BOOST, so thatdischarge device TD is turned off before the boost is initiated. Athigher VDD voltages, the relationship between signal WSELP<0> and signalWSELP<1> changes. The amount of separation between signals WSELP<0:1>determines the boosting value.

There are several disadvantages associated with the SRAM write assistcircuit 300. For example, the SRAM write assist circuit 300 is based oncomplex logic implementations using analog techniques that work on smalldelay variations, which can disadvantageously vary on the silicon.Further, in instances in which bias temperature instability causessignificant threshold voltage shifts in the FETs, the delays may beshifted and can behave differently such that either full attenuation isattained (concern for writability) or no attenuation is attained(concern for reliability). Consequently, yield variation forsemiconductor devices can be observed with these techniques.

FIG. 5 shows a SRAM write driver 500 according to embodiments of thepresent invention that addresses the disadvantages associated with theSRAM write driver 200 of FIG. 2. In embodiments, the SRAM write driver500 is coupled to the bit line pairs (e.g., bit line true and bit linecomplement) represented by bit line 115 depicted in FIG. 1 via bitswitches (not shown). The bit line true may be formed from one or moretrue bit lines and the bit line complement may be formed from one ormore complement bit lines. The bit switches, which are used to connectone of the bit lines (e.g., bit line true or bit line complement) to theSRAM write driver 500, are formed from N-type field effect transistors(NFETs) with gates connected to the one or more true bit lines for bitline true and similar NFETs with gates connected to the one or morecomplement bit lines for bit line complement. Note that cross-coupledinverters described above for an SRAM cell would be coupled to each ofthe bit lines.

In a typical write operation, after a particular word line has beenactivated, one of the bit lines (e.g., bit line true) is brought toground (GND), while the other bit line (e.g., bit line complement) ismaintained at a supply voltage (VDD). This discharges the cell andfacilitates a write to the cell. In particular, this enables the “1”side (e.g., bit line true) to change to a “0”, allowing the cell to flipstates. As shown in FIG. 5, a discharge device TD (an NFET) and a boostcapacitor C_BOOST are used to discharge one of the bit lines (e.g., bitline true or bit line complement). Capacitor C_BOOST is coupled todischarge device TD at a node WBOOST (e.g., a negative boost node) andcontrolled by signal WSELP<1>. The gate of discharge device TD, which iscontrolled by signal WSELP<0>, is coupled to capacitor C_BOOST via NFET505 and PFET 510. Discharge device TD and capacitor C_BOOST areconnected to bit switches, bit line true, and bit line complement viaNFETs 515 and 520, which are controlled by data signals WGDLT and WGDLC,respectively.

In embodiments, the SRAM write driver 500 further comprises NFET 525.The drain of NFET 525 is connected to the gate of discharge device TD,the source of NFET 525 is connected to array supply voltage (VCS), andthe gate of NFET 525 is connected to WSELP<0>. The source of PFET 505 isconnected to VCS, the drain of PFET 510 is connected to the gate ofdischarge device TD, and the gate of PFET 505 is connected to WSELP<0>.The source of NFET 505 is connected to voltage source (VSS) (e.g., 0 v)instead of the WBOOST node (e.g., as shown in FIG. 2), the drain of NFET505 is connected to the gate of discharge device TD, and the gate ofNFET 505 is connected to WSELP<0>. Consequently, NFET 505, NFET 525, anddischarge device TD are configured as high voltage devices and capacitorC_BOOST is configured as a low voltage device. The discharge device TDacts like a diode and functions to clamp WBOOST from going very negative(e.g., a BOOSTMIN of about −250 mv). The NFET 525 competes with the NFET505 and functions to clamp the n-channel metal-oxide-semiconductorfield-effect transistor (NMOS) disable (NDIS) signal or voltage. Theclamping of the NDIS signal is controllable and allows for attenuationof WBOOST in accordance with aspects of the present invention, asdiscussed in detail herein.

Advantageously, in additional or alternative embodiments, theconfiguration of NFET 505, PFET 510, and NFET 525 (e.g., a clampingdevice) can be moved out of the SRAM write driver 500 and into a writeassist attenuation circuit 600 depicted in FIG. 6. In particular, FIG. 6shows NFET 505 and PFET 510 depicted as inverter 605 (e.g., an invertercomprising PFET and NFET) and NFET 525 depicted as NFET 610, NFET 615,or NFET 620 (e.g., a clamping device that can be programmed) within thewrite assist attenuation circuit 600. The source of NFET 610 isconnected to WSELP<0>, the drain of NFET 610 is connected to VCS, andthe gate of NFET 610 is connected to signal EMA00. The source of NFET615 is connected to WSELP<0>, the drain of NFET 615 is connected to VCS,and the gate of NFET 615 is connected to signal EMA01. The source ofNFET 620 is connected to WSELP<0>, the drain of NFET 620 is connected toVCS, and the gate of NFET 620 is connected to signal EMA10.

In embodiments, signal WSELP<0> is processed by OR gate 625 to generatea pulsed signal that is further processed by OR gates 630, 635, and 640ORing with EMA00_N, EMA01_N and EMA10_N (generation of EMA00_N, EMA01_Nand EMA10_N is not shown in FIG. 6) to generate signals EMA00, EMA01,and EMA10, respectively. As such, the pulse signal generated by the ORgate 626 is added onto signals EMA00, EMA01, and EMA10, which enablesclamping of the NDIS signal until and during the boosting and allows forthe NDIS signal to go to GND after the boosting, as discussed in furtherdetail hereafter with respect to FIGS. 9, 10A, 10B, and 11.

Each of the signals EMA00, EMA01, and EMA10 are operable to be inputinto the NFETs 610, 615, and 620, respectively, to control which NFET isactivated. The channel widths of each of the NFETs 610, 615, and 620 arefabricated differently (e.g., the channel width may be setup such thatNFET 615<NFET 610<NFET 620) in order to provide different leakagecurrent, and thus control clamping of the signal WSELP<0> at differentvoltages allow for attenuation of WBOOST. In this manner, attenuationwith clamping proportional to supply voltage and processes givesexponentially increased attenuation because of larger leakage, whichadvantageously provides a tunable negative boost write assistattenuation circuit that can either reduce attenuation or increaseattenuation at particular voltages.

FIG. 7 shows a attenuation table 700 that illustrates functionality ofsignals EMA00, EMA01, and EMA10 to individually activate the NFETs 610,615, and 620 depicted in FIG. 6 in accordance with aspects of thepresent invention. In particular, FIG. 7 shows that EMA00 may beconfigured to activate NFET 610, which in embodiments has a channelwidth configured to clamp the NDIS signal in such a manner that defaultattenuation of WBOOST is achieved. EMA01 may be configured to activateNFET 615, which in embodiments has a channel width configured to clampthe signal WSELP<0> in such a manner that lesser attenuation of WBOOSTis achieved. EMA10 may be configured to activate NFET 620, which inembodiments has a channel width configured to clamp the signal WSELP<0>in such a manner that larger attenuation of WBOOST is achieved. Lastly,EMA11 may be configured to activate the NFET 610 with defaultattenuation along with pushed out Boost timing.

Advantageously, in embodiments, the moving of NFET 505, PFET 510, andNFET 525 depicted in FIG. 5 to the write assist attenuation circuit 600depicted in FIG. 6 as inverter 605 and NFETs 610, 615, and 620 frees upsemiconductor area within the SRAM write driver 800, as depicted in FIG.8. In particular, FIG. 8 shows that the signal WSELP<0> modified byeither of NFET 610, 615, or 620, or WSELP<0> asserted withoutmodification from either of NFETs 610, 615, and 620 is directly inputinto the gate of discharge device TD. Accordingly, the discharge deviceacts like a diode and functions to clamp WBOOST dependent upon either ofsignals EMA00, EMA01, EMA10, or EMA 11.

FIG. 9 shows a simulation 900 of the discharge of a bit line (BLT) withthe write assist attenuation circuit 600 depicted in FIG. 6. Inparticular, simulation 900 shows the transition of a bit line (BLT) fromhigh to low, and then to the point where it receives a boost (WBOOST)below GND (e.g., 0 volts). As shown in FIG. 9, in contrast toconventional schemes where the NDIS signal follows BLT below GND, inembodiments of the present invention, the NDIS signal is clamped aboveGND, which results in attenuation of WBOOST similar to that ofconventional attenuation discussed herein based on timing of theWSELP<0:1>. This happens because the write assist attenuation circuit600 is controlling the signals EMA00, EMA01, EMA10, or EMA11 to modifyor not modify WSELP<0>.

FIGS. 10A and 10B show low voltage simulation 1000 and high voltagesimulation 1100, respectively, of the discharge of a bit line (BLT) withthe write assist attenuation circuit 600 depicted in FIG. 6. Inparticular, simulations 1000 and 1100 show the transition of a bit line(BLT) from high to low, and then to the point where it receives a boost(WBOOST) below GND (e.g., 0 volts). As shown in FIG. 10A, the NDISsignal is variably clamped above GND, which results in attenuation ofWBOOST to a BOOST_MIN of −208 mv compared to a BOOST_MIN of −239 mvachieved by a conventional scheme for low voltage writing. As shown inFIG. 10B, the NDIS signal is variably clamped above GND, which resultsin attenuation of WBOOST to a BOOST_MIN of −303 mv compared to aBOOST_MIN of −394 mv achieved by a conventional scheme for high voltagewriting.

FIG. 11 shows a simulation 1200 of the discharge of a bit line (BLC)with the write assist attenuation circuit 600 depicted in FIG. 6. Inparticular, simulation 1200 shows the transition of a bit line (BLC)from high to low, and then to the point where it receives a boost(WBOOST) below GND (e.g., 0 volts). As shown in FIG. 11, the clamping ofthe NDIS signal is controllable via the write assist attenuation circuit600 such that clamping only occurs during the boost (WBOOST) and returnsthe NDIS signal to GND after application of the boost (WBOOST).Advantageously, in embodiments, this controlled clamping results in noDC current being used while bit line (BLC) returns to a high state, andthus achieves significant power savings during each write cycle to amemory array.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method, comprising: forming a memory arraycomprising a plurality of static random access memory (SRAM) cells;forming a plurality of true bit lines each connected to a column of thememory array; forming a plurality of complement bit lines each forming adifferential pair with, and in a same column as, one of the plurality oftrue bit lines; forming a write driver connected to each of thedifferential pair of bit lines in each of the plurality of SRAM cells ofthe memory array, the write driver comprising: a negative boost node; adischarge device coupled to ground and the negative boost node, thedischarge device configured to receive a control signal and pull one ofthe plurality of true bit lines or one of the plurality of complementbit lines to ground in an active phase of a write cycle; and a boostcapacitor coupled to the negative boost node, the boost capacitorconfigured to boost the one of the plurality of true bit lines or theone of the plurality of complement bit lines below ground; and forming awrite assist attenuation circuit connected to the discharge device, thewrite assist attenuation circuit comprising a clamping device comprisingan inverter, a first NFET, a second NFET, and a third NFET, each havinga source-drain path connected to an output of the inverter whichsupplies the control signal to the source-drain paths of the first NFET,second NFET and third NFET, respectively, the clamping device beingconfigured to modify the control signal as a function of supply voltageand process to attenuate an amount of the boost, and each of the firstNFET, the second NFET, and the third NFET being connected to a firstattenuation signal, a second attenuation signal, and a third attenuationsignal, respectively, wherein the source-drain paths of the first NFET,the second NFET and the third NFET are connected in parallel with oneanother between the supply voltage and the output of the inverter. 2.The method of claim 1, further comprising forming a source of the firstNFET connected to the control signal, and forming a drain of the firstNFET connected to an array supply voltage, and forming a gate of thefirst NFET connected to the first attenuation signal.
 3. The method ofclaim 2, further comprising forming a source of the second NFETconnected to the control signal, forming a drain of the second NFETconnected to the array supply voltage, and forming a gate of the secondNFET connected to the second attenuation signal.
 4. The method of claim3, further comprising forming a source of the third NFET is connected tothe control signal, forming a drain of the third NFET connected to thearray supply voltage, and forming a gate of the third NFET connected tothe third attenuation signal.
 5. The method of claim 4, wherein a widthof the channel of each of the first NFET, the second NFET, and the thirdNFET is different such that each of the first NFET, the second NFET, andthe third NFET modifies the control signal in a different manner.
 6. Themethod of claim 5, wherein the width of the channel of the second NFETis less than the width of the channel of the first NFET, and the widthof the channel of the first NFET is less than the width of the channelof the third NFET.
 7. A method, comprising: forming a memory arraycomprising a plurality of memory cells; forming a plurality of true bitlines each connected to a column of the memory array; forming aplurality of complement bit lines each forming a differential pair with,and in a same column as, one of the plurality of true bit lines; forminga write driver connected to each of the differential pair of bit linesin each of the plurality of memory cells of the memory array, the writedriver comprising: a negative boost node; a discharge device coupled toground and the negative boost node, the discharge device configured toreceive a control signal and pull one of the plurality of true bit linesor one of the plurality of complement bit lines to ground; and a boostcapacitor coupled to the negative boost node, the boost capacitorconfigured to boost the one of the plurality of true bit lines or theone of the plurality of complement bit lines below ground; and forming awrite assist attenuation circuit connected to the discharge device, thewrite assist attenuation circuit comprising a clamping device comprisinga first NFET, a PFET, and a second NFET, forming a source of the firstNFET being connected to a voltage source, a drain of the first NFETbeing connected to the discharge device, and a gate of the first NFETbeing connected to the control signal, forming a source of the PFETbeing connected to an array supply voltage, a drain of the PFET beingconnected to the discharge device, and a gate of the PFET beingconnected to the control signal, and forming a source of the second NFETbeing connected to the array supply voltage, and a drain of the secondNFET being connected to the discharge device, wherein the PFET and thesecond NFET are connected in parallel to one another between the arraysupply voltage and the discharge device, and a gate of the second NFETbeing connected to the control signal, wherein a source-drain path ofthe second NFET is connected between the discharge device and a commonnode connecting source-drain paths of the first NFET and the PFET. 8.The method of claim 7, wherein: the memory cells are static randomaccess memory cells, the discharge device is comprised of an NFETconfigured to act like a diode, and the source-drain paths of the firstNFET, the PFET and the second NFET are commonly connected to a gate ofthe NFET comprising the discharge device.
 9. The method of claim 8,wherein the discharge device is configured to pull one of the pluralityof true bit lines or one of the plurality of complement lines to groundin an active phase of a write cycle.
 10. The method of claim 9, whereinthe boost capacitor is coupled to a drain of the NFET comprising thedischarge device.